TAU (Thousand Astronomical Unit) mission is an interstellar precursor mission concept, studied by JPL scientists in the late 1980s, with the potential for enabling an unmanned probe to reach a distance of 1,000 astronomical units (AU) (0.016 light-years) within a 50-year trip time. TAU would be powered by a nuclear reactor in the 1-MWe class with a specific mass of 12.5 kg/kWe and a full-power operating (thrusting) time of 10 years. RTG power supplies would provide power for the science instruments for the 40-year cruise portion of the mission (after reactor and propulsion system burn-out and jettison). The electric thrusters required would have a specific impulse (I_sp of 10,000–20,000 lbf-s/lbm) and a burn time (per thruster) of 2 years, as might be available from advanced ion thrusters. The propellant would be xenon (see XIPS). The propulsion system (ion thrusters, power processor units, etc.) specific mass would be 4 kg/kWe.

The primary science objective of the mission is to measure directly the distance to stars throughout our galaxy using stellar parallax. Secondary science objectives include particles and fields measurements, a search for the heliopause, a search for the Oort Cloud, tests of gravitational effects based on changes to the spacecraft's trajectory (which could be caused by a tenth planet or other "dark" companions of the solar system), and tests of general relativity (such as using the optical communications link with the spacecraft as a long-baseline gravity wave detector).